During continuous pelletizing operations, a liquid at increased pressure generally flows through the pelletizing chamber. The liquid flowing through the chamber is generally water. The pelletizing process is therefore also termed underwater pelletizing.
By way of example, the underwater pelletizing process is used when pellets are produced from plastics which can still comprise residual monomer, water, or other low-boiling substances through the production process. This can lead to foaming of the plastic during pelletization at ambient pressure. By virtue of the increased pressure in the pelletizing chamber, expansion of the plastic during the pelletizing process is prevented.
A process for production of expandable plastics pellets is described by way of example in EP-A 0 305 862. In that process, a polymer base material or a polymer mixture is fed to an extruder and melted in the extruder. The extruder has an injector for addition of a blowing agent to the melt. This blowing agent is added under pressure. The melt, with the blowing agent dissolved therein, is pelletized in a pelletizing chamber through which water flows. The pellets are entrained by the stream of water and introduced into a dryer in which the pellets are dried. Examples of polymer compositions mentioned as suitable are aromatic alkenyl polymers or copolymers, e.g. polystyrene, styrene-maleic anhydride copolymer, polycarbonate, polyester, polyetherimide, polysulfone, and polyphenyl ether.
From polyamide preparation, it is known that water arising during the poly-condensation of dicarboxylic acid with diamine dissolves in the polyamide. This water leads to foaming of the polyamide during pelletization without devolatilization. Large undesired bubbles can also arise in the pellets. The current method of devolatilization uses a vented extruder which necessitates high capital expenditure and high maintenance costs, or a separator, which rapidly suffers from encrusting and then has to be cleaned out.